In some cases, predetermined rotational unbalance is intentionally left unremoved, i.e. preset, on rotating members, such as crankshafts, when adjusting dynamic rotational balance conditions of the rotating members, for the following reasons. In the case of a crankshaft of an engine, for example, inertial force of the crankshaft would not appropriately balance as a whole even if the crankshaft itself has been adjusted independently into perfect rotational balance, due to the fact that pistons, connecting rods, etc. are connected with the crankshaft. Such unbalanced inertial force would cause great oscillation of the crankshaft during rotation about the crankshaft's rotation axis. To address the inconvenience, it has been known to make balance adjustment of a crankshaft such that predetermined appropriate rotational unbalance is intentionally left or preset on the crankshaft so that, with the pistons etc. connected with the crankshaft, the inertial force caused by the rotational unbalance of the crankshaft can cancel out a portion of inertial force caused by the reciprocating mass of the pistons etc.
Roughly classified, there have been three major schemes for adjusting a dynamic rotational balance condition of a crankshaft such that predetermined rotational unbalance is intentionally left or preset on the crankshaft for smooth rotation.
According to the first adjustment scheme, dummy rings of given weight values, previously measured or calculated using a predetermined reference crankshaft, which correspond to the predetermined rotational unbalance to be preset on the crankshaft, are attached to the individual crankpins of the crankshaft, and an initial rotational unbalance value of the crankshaft is determined by detecting a balanced state of the crankshaft rotating about its rotation axis while measuring a balance condition of the crankshaft by means of a rotational balance testing apparatus. Then, a correction amount and angular position of the rotational unbalance are determined.
According to the second adjustment scheme, a spindle is connected to a crankshaft on an oscillatable measuring table, a dummy weight equivalent to a previously calculated value, which corresponds to predetermined rotational unbalance to be preset on the crankshaft, is attached to the spindle, and an initial rotational unbalance value is determined by balancing the spindle in synchronism with the crankshaft. Then, a correction amount and angular position of the rotational unbalance are determined.
Further, according to the third adjustment scheme, a crankshaft is rotated about its rotation axis alone with no dummy ring or dummy weight attached thereto, and a value of rotational unbalance caused during the rotation is detected by oscillation pickups as analog oscillation signals. The analog oscillation signals are converted via A/D (Analog-to-Digital) converters into digital signals, and then the digital detected values of the rotational unbalance are subtracted from previously-known dummy weight values, represented by digital dummy information, to thereby determine an initial rotational unbalance value. Then, a correction amount and angular position of the rotational unbalance are determined.
Generally, actual rotational balance adjustment of the crankshaft after the aforementioned measurement of the rotational balance condition is performed by drilling holes of optimal depths in optimal positions of counterweights of the crankshaft on the basis of the rotational unbalance correction amount and angular position determined in the above-described manner.
However, the above-mentioned first adjustment scheme requires attachment/detachment of the dummy rings for each crankshaft to be measured. Further, where one and the same apparatus is used to measure rotational balance conditions of many models of crankshafts, it is necessary to prepare a different set of dummy rings for each of the models and replace the dummy ring set whenever a changeover is to take place from one crankshaft model to another. For these reasons, the first adjustment scheme necessitates preparation of various types of dummy rings and encounters considerable difficulties in automatizing the rotational balance condition measurement.
The above-mentioned second and third adjustment schemes, on the other hand, can eliminate the need for attachment/detachment of dummy rings for each crankshaft to be measured. However, if a crankpin to be tested has rotational phase errors beyond an allowable range, the measured results would undesirably contain great errors so that unsatisfactory balance adjustment, incapable of appropriately removing undesired rotational unbalance, tends to be performed on the crankshaft.